# EPR doubts

1. Jun 12, 2012

### the_pulp

Sorry, I have 2 doubts about the EPR paradox.

1) Just check out if Im ok: What EPR proposed was that QM suggest that if two entagled particles, for example photons, prepaired in some special way, get away from each other then:
a) if we measure their x-spin then one particle will get the value "a" and the other "-a" (a=+-1)
b) if we measure their y-spin then one particle will get the value "a" and the other "-a" (a=+-1)
c) if we measure their z-spin then one particle will get the value "a" and the other "-a" (a=+-1)
This is weird because there seams to be an instantanous communication between the two particles which violates locallity (I know that the hidden variable hipothesys is not valid because of Bell inequalities and all that so the only possibility is the violation of locality)
2) Has anybody ever made any experiment with two EPR entangled particles and demostrated that the world effectively works this way? Because I see this instantanous
communication ridiculous. Isnt it possible that this doesnt occur and that if we measure some property on both particles, this results are not this perfectly correlated???

2. Jun 12, 2012

### Staff: Mentor

Experiments related to Bell's Theorem use entangled particles (usually photons). Look at some of the many Bell's Theorem threads in this forum, and follow the references to experiments.

3. Jun 12, 2012

### stevendaryl

Staff Emeritus
The predictions of quantum mechanics seem to be born out by experiment.

However, it's a little slippery to try to prove that the correlations involve faster-than-light communication. The rule is: (For pairs of spin-1/2 particles)

If one experimenter (Alice) measures x-spin, AND she gets spin-up, then the other experimenter (Bob) will get spin-down for a measurement of x-spin. Alice's result (spin-up or spin-down) is completely random, so Alice can't use this correlation to force Bob to see spin-up. The correlation can't be used to communicate; nothing that Bob sees gives him any indication of which axis Alice used.

4. Jun 13, 2012

### martinbn

I am confused about this myself. But the way I understand it, to say that there is instantaneous communication of some sort you need the two events, the click (or whatever the apparatus does when a measurement is performed) at Alice and the one at Bob, to be in cause effect relation. All we have is that the events a correlated, and correlation and causation are different things.

5. Jun 13, 2012

### the_pulp

But has it ever been done? I mean, seing one particle x-spin in Shangai and the other particle x-spin in Oslo and notice the correlation? Because I think that it is impossible! Ive seen the math in wikipedia, but I think that if we add the principle of Locality, as in QFT, it is imposible to make such an entagled-distant state of two particles.

Thanks!

6. Jun 13, 2012

### f95toli

Yes, it has been done lots of times. Not in Oslo and Shanghai, but certainly at distances large enough (100s of miles) to prove the effect (one limitation is that you need to be able to connect the two setups with a single optical fibre, this limits the distances a bit).
I think Zeilingers group has been involved in a number of these experiments.

7. Jun 13, 2012

### DrChinese

You are correct:

http://arxiv.org/abs/quant-ph/9810080

Violation of Bell's inequality under strict Einstein locality conditions
Gregor Weihs, Thomas Jennewein, Christoph Simon, Harald Weinfurter, Anton Zeilinger (University of Innsbruck, Austria)

Abstract: "We observe strong violation of Bell's inequality in an Einstein, Podolsky and Rosen type experiment with independent observers. Our experiment definitely implements the ideas behind the well known work by Aspect et al. We for the first time fully enforce the condition of locality, a central assumption in the derivation of Bell's theorem. The necessary space-like separation of the observations is achieved by sufficient physical distance between the measurement stations, by ultra-fast and random setting of the analyzers, and by completely independent data registration. "

I can't find the reference right now, but one Bell test went between islands IIRC.

8. Jun 13, 2012

### DrChinese

Ah, here it is:

http://arxiv.org/abs/quant-ph/0607182

Free-Space distribution of entanglement and single photons over 144 km
R. Ursin, F. Tiefenbacher, T. Schmitt-Manderbach, H. Weier, T. Scheidl, M. Lindenthal, B. Blauensteiner, T. Jennewein, J. Perdigues, P. Trojek, B. Oemer, M. Fuerst, M. Meyenburg, J. Rarity, Z. Sodnik, C. Barbieri, H. Weinfurter, A. Zeilinger (2006)

9. Jun 13, 2012

### DrChinese

This is something of a matter of interpretation. What is certain is that it is "quantum non-local" (what you define that to be).

For example: if the future can affect the past (i.e. assuming time symmetry), then locality can be maintained. In this view, it is possible to explain the results by considering the future measurement context as being an input variable. But now you are sacrificing classical causality, which you may find just as ridiculous.

Basically, you have to give up at least one prized possession in this deal: locality, causality, realism, etc.

10. Jun 13, 2012

### the_pulp

So is there any way / interpretation in which we dont have to give up locality and causality??

Thanks for your answers, this short thread corrected one big mistake of mine!

11. Jun 14, 2012

### chogg

Two I know of are: Consistent Histories, and the Everett relative-state interpretation (a.k.a. "Many Worlds"). Consistent Histories is explained thoroughly in a book which is now freely available online: http://quantum.phys.cmu.edu/CQT/. I don't know what resources people use for Many Worlds.

With Consistent Histories, the price you pay is "unicity": the idea that there exists a single ultimate description of reality. (Instead, you are free to choose from multiple possible descriptions, all equally viable, but some of which cannot be compared to each other.)

With "Many Worlds", the price you pay is the idea that only one outcome actually occurs. (Instead, reality branches.)

Quantum mechanics forces us to give up cherished ideas about the universe. It is often claimed that QM disproves local realism -- not so! As Dr. Chinese pointed out, there are other viable options. FWIW, I share your view: I just couldn't understand a universe without local realism. The interpretations I mentioned above let us keep it.

12. Jun 14, 2012

### DrChinese

You have to scan the various interpretations and decide for yourself. It is generally said that the Many Worlds Interpretation (MWI) is local but not realistic. (Realistic in this context means that quantum observables have well-defined values independent of the act of observation.) But I don't believe it can be labeled causal.

A lot of it comes down to the definition of words. I would say the following 2 statements are generally accepted by most physicists:

a) An entangled pair of photons CANNOT be considered 2 separate and independent objects while they are entangled, and therefore exhibit quantum non-local properties when spacelike separated.
b) Quantum observables do NOT have well-defined values independent of the context of a measurement.

The various interpretations wrap somewhat different descriptions around these statements.

13. Jun 14, 2012

### harrylin

I guess that, as also DrC and chogg suggested, all answers that assume that QM is perfectly correct have some kind of "ridiculous" consequence.

There is also still the possibility that QM will be found to fail for experiments that don't have any remaining flaw or loophole, or that it will be found that such an experiment is not possible. However, recently some experiments have been done of which the experimenters claim that they are free of such (will be interesting to discuss!).

14. Jun 15, 2012

### the_pulp

Thanks to all of you for such valuable answers. Just one more question. arent this two afirmations incompatible between them:

1) EPR: If you know that in one moment the property of a particle in Shangai takes the value "a" then you are sure that the value that the same property of the other particle in Oslo takes, at the same time, the value "-a".
2) Conmutation relation in QFT: [phi(Shangai,t);phi(Oslo,t)]=0

So EPR disproves QFT????

Thanks!!!!

15. Jun 15, 2012

### vanhees71

You shouldn't thank for being mislead ;-). QFT is one form of quantum theory and thus quantum phenomena like entanglement cannot contradict it.

If you have entangled states of the kind described in this thread and measure one property on one subsystem, that's (according to QFT) a local interaction at the place of the detector. That's so, because QFT is a local theory.

That at the moment of reading the outcome of this meassurement also the entangled variable of the other subsystem is known to have a certain value is not caused by the measurement but by the fact that the system has been prepared in an entangled state of two subsystems. So there is no "spooky interaction" involved due to the measurement process.

To this conclusion you can only come if you think in terms of Copenhagen-interpretation like state-collapse ideas, which in this way contradict locality in the QFT sense and that's why one should not think in these terms, as has been pointed out by Einstein, Podolsky, and Rosen in their famous paper, arguing against the completeness of quantum mechanics. In fact, it's only a paper against the unnecessary assumption of a mysterious mechanism called "collapse". Using the Minimal Statistical Interpretation of quantum theory, there is no more mystery left.

Whether quantum theory is a complete description of nature or not is not implied by this statement. In fact we can never know whether any theory of physics is complete. We can only say that today there are no empirical facts disproving quantum mechanics and thus that it is unlikely that quantum mechanics is fundamentally wrong, but whether it really describes accurately all there is in nature, can never be proven. This is true for any physical theory.

16. Jun 15, 2012

### harrylin

That's a very interesting take! Most experts seem to favour some kind of magical or weird looking interpretation, such as "spooky action at a distance". It's largely because of such mysterious interpretations that QM is said to be "not understood". You mentioned it now a few times, but is there a thread on Minimal Statistical Interpretation of quantum theory?

17. Jun 15, 2012

### vanhees71

I don't know, whether there is a thread. I think with the Minimal Interpretation all these troubles with esoteric conclusions about action at a distance etc. are simply not there, and since the Minimal Interpretation is sufficient (and also necessary since it's minimal ;-)) to make predictions from quantum mechanics about observations, I don't see why I should use any other interpretation.

18. Jun 17, 2012

### harrylin

Funny enough, not only I know nothing about it, it is also not mentioned (at least not with that name) in the rather recent poll about interpretations: